1. Field of the Invention
The present invention relates to making, in monolithic form, bidirectional switches of medium power.
2. Discussion of the Related Art
The most current static bidirectional switches are triacs. A triac corresponds to the antiparallel association of two thyristors. It can thus be directly connected in an A.C. network, for example, the mains. The gate of a conventional triac corresponds to the cathode gate of one at least of the two thyristors forming it and is referenced to the electrode located on the front surface of this triac, that is, the surface that includes the gate terminal, while the other triac surface is typically connected to a heat sink and to the ground.
Bidirectional switches of the type described in European patent application No. 0817277, the triggering of which is ensured by applying a voltage between a control electrode located on the front surface of the component and a main electrode located on the opposite surface of the component, will more specifically be considered hereafter.
FIG. 1 shows an equivalent electric diagram of such a bidirectional switch. A control electrode G of the bidirectional switch is connected to the emitter of a bipolar transistor T, the collector of which is connected the anode gates of first and second thyristors Th1 and Th2 placed in antiparallel between two terminals A1 and A2. Terminal A1 corresponds to the anode of thyristor Th1 and to the cathode of thyristor Th2. Terminal A1 is also connected to the base of transistor T. Terminal A2 corresponds to the anode of thyristor Th2 and to the cathode of thyristor Th1.
FIG. 2 is a simplified cross-section view of an example of monolithic embodiment of the bidirectional switch described in relation with FIG. 1. Transistor T is formed in the left-hand portion of the drawing, thyristor Th1 at the center, and thyristor Th2 to the right thereof.
The structure of FIG. 2 is formed from an N-type lightly-doped semiconductor substrate 1. The anode of thyristor Th1 corresponds to a P-type layer 2 that is formed on the rear surface side of substrate 1. Its cathode corresponds to an N-type region 3 formed on the front surface side in a P-type well 4. The anode of thyristor Th2 corresponds to a P-type well 5 formed on the front surface side and its cathode corresponds to an N-type region 6 formed on the rear surface side in layer 2. The periphery of the structure is formed of a heavily-doped P-type layer 7 extending from the front surface to P-type layer 2. Conventionally, region 7 is obtained by drive-in from the two substrate surfaces. The rear surface is coated with a metallization M1 corresponding to first terminal A1 of the bidirectional switch. The upper surfaces of regions 3 and 5 are coated with a second metallization M2 corresponding to second terminal A2 of the bidirectional switch. An N-type region 8 is formed, on the front surface side, in a P-type well 9 in contact with peripheral region 7. The surface of region 8 is contacted by a metallization M3 connected to control terminal G of the bidirectional switch. A metallization M4 may be formed on the upper surface of peripheral region 7. Metallization M4 is not connected to an external terminal. As an alternative, well 9 may be separated from peripheral region 7 and electrically connected thereto via metallization M4.
The operation of this bidirectional switch is the following.
When terminal A2 is negative with respect to terminal A1, thyristor Th1 is likely to be on. If a sufficiently negative voltage with respect to metallization M1 is applied to gate G, the base-emitter junction of transistor T is forward biased and this transistor turns on. A vertical current ic shown in dotted lines in FIG. 2 thus flows from metallization M1, through the forward junction between layer 2 and substrate 1, then into regions 1, 9 and 8 corresponding to transistor T. Carriers are thus generated at the level of the junction between substrate 1 and well 9 near the junction between substrate 1 and well 4, and thyristor Th1 is turned on. It can also be considered that an auxiliary vertical NPNP thyristor including regions 8-9-1-2, region 9 of which forms the cathode gate region, has been triggered.
Similarly, when terminal A2 is positive with respect to terminal A1, applying a negative voltage on terminal G turns transistor T on. The carriers present in the vicinity of the junction between substrate 1 and layer 2 turn thyristor Th2 on, as will be better understood by referring to the simplified top view of FIG. 4 in which it can be seen that the region corresponding to transistor T is a neighbor to a portion of each of thyristors Th1 and Th2.
Practice reveals that this type of bidirectional switch has a non-optimal control responsiveness, that is, especially, that the current required to trigger thyristor Th1 is of several hundreds of milliamperes.
An object of the present invention is to provide a novel embodiment in monolithic form of a bidirectional switch of the above mentioned type that exhibits a greater control responsiveness of thyristor Th1.
To achieve this and other objects, the present invention provides a monolithic bidirectional switch formed in a semiconductor substrate of a first conductivity type having a front surface and a rear surface, including a first main vertical thyristor, the rear surface layer of which is of the second conductivity type, a second main vertical thyristor, the rear surface layer of which is of the first conductivity type, an auxiliary vertical thyristor, the rear surface layer of which is of the second conductivity type and is common with the rear surface layer of the first main thyristor, a peripheral region of the second conductivity type especially connecting the rear surface layer of the auxiliary thyristor to the layer of this thyristor located on the other side of the substrate, a first metallization on the rear surface side, a second metallization on the front surface side connecting the front surface layers of the first and second thyristors. An additional region isolates the rear surface of the auxiliary thyristor and the first metallization.
According to an embodiment of the present invention, the additional region is made of a semiconductor material of the first conductivity type.
According to an embodiment of the present invention, the thickness of the additional region is smaller than that of the rear surface region of the second main vertical thyristor.
According to an embodiment of the present invention, the additional region is made of silicon oxide.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.